1. Field of the Invention
The present invention relates to data communication with subscriber stations in radio communications networks. To be more precise, the invention relates to the problem of joint use of radio resources by a number of central stations in a radio communications network such as this.
2. Description of the Related Art
Access methods (Multiple Access, MA) which define the right of an individual station to access the medium are used in order to allow a number of stations to access a shared transmission medium in communications systems of any desired type. A distinction is in this case drawn as to whether the medium is subdivided in the time domain (Time Division Multiple Access TDMA), in the frequency domain (FDMA), in the code domain (CDMA) or in the space domain (SDMA) between the stations.
If the joint use takes place in the time domain, there is a problem in defining the time period during which one station has exclusive access to the transmission medium, in order to avoid collisions occurring. If a number of stations access the shared medium in an uncoordinated manner, collisions can frequently occur as soon as more than one station is transmitting at the same time. If a central station (for example a base station) monitors the time access for a group of stations (for example mobile stations), collisions can be prevented for these stations.
If the aim is to supply an area which is larger than the maximum range of a central station, a number of central stations must be used. These central stations are normally assigned different frequency bands for communication with subscriber stations which are within range of them, in order in this way to avoid collisions between the accesses by the central stations. If a supply area is subdivided into cells which are supplied by fixed-position stations (base stations), and base stations may use only a limited number of frequency bands, this results in a cellular radio system.
The number of frequency bands which are available for a radio communications system is limited. This means that there must be a number of central stations in a radio communications network having a large number of cells, sharing the use of one and the same frequency band. Efforts are normally made to arrange the cells geographically such that cells which use the same frequency band are sufficiently far away from one another that radio signals which are transmitted in one of these cells no longer interfere with communication in another which is operating at the same frequency.
As the density of subscriber stations rises continuously and the communication traffic in the mobile radio networks rises in a corresponding manner, this principle reaches its limits, however, since the number of connections which can be handled simultaneously using a given frequency band, and hence the number of subscribers who can be controlled simultaneously in one cell, are limited. In order to increase the transmission capacity, it would admittedly be possible to consider dividing a cell and assigning different frequency bands for each of the two resultant cell elements; however, this generally leads to the problem of the frequency which is to be used in one of the newly created cells already being used by another cell in the vicinity, which is not sufficiently far away to make it possible to preclude mutual interference.
Similar problems can occur if it is impossible to supply all the locations within a given cell with a radio signal of sufficient intensity to allow problem-free mobile communication. Since the transmission power of a central station cannot be increased without running into the risk of causing interference in other cells that are operating at the same frequency in the radio communications system, and the transmission power of the subscriber stations, which are generally operated independently of the network, is restricted in any case, an improvement in the signal supply can be achieved only by “sharing” the central station between two different locations within the cell, with the difficulty that the “station elements” at the different locations must coordinate their radio traffic in order to avoid interfering with one another.
One approach for solving the problem of coordination of base stations which use the same frequency band has been proposed in DE 198 24 961 A1. In this known method, the transmission frame for one TDMA radio signal is subdivided into a number of containers, with one container representing a specific number of time slots in the TDMA frame, and the containers each being allocated for use by different base stations. One base station is therefore not allocated all the time slots in the frame but only a subset of them, which it can in each case allocate on the basis of any desired known method for communication with terminals.
One such method is suitable for communication when the amount of transmission traffic is constant, in particular for speech communication; however, there are problems in using this for data communication, where the requirements for the time response relating to the transmission are generally less stringent but where considerable amounts of data frequently need to be transmitted in short time periods. The capability to make use of packets which remain free in a synchronous channel for speech transmission for data transmission is restricted since each central station may have access only to those synchronous channels which correspond to the containers allocated to it. It is often uneconomic, or impossible, to allocate additional containers to a central station which has to transmit a large amount of data at short notice, since all the containers that are available within the frame are already being used by one central station.
Modern mobile radio standards which are also designed for mobile data communication, such as GPRS or HIPERLAN/2, no longer use the concept of synchronous channels which are predetermined by fixed time slots in a transmission frame and, instead of this, use frames whose structure is not predetermined in a fixed manner, but which each contain format information which provides a connected receiver with information about the format of the respective current frame and which, in particular, defines the location of uplink and downlink phases in the frame. In a system such as this, subscriber stations and a central station can each interchange different amounts of data from one frame to the next; a specific predetermined fraction of the transmission capacity, which does not vary with time, is no longer reserved for communication with a specific subscriber station and, instead, each subscriber station uses from one frame to the next precisely the transmission capacity which it requires, and transmission capacity which is not required for time-critical services such as speech transmission is completely available for asynchronous data transmission.